This invention relates to a process for treatment of ores and scrap materials for the extraction of precious metals and heavy minerals and more particularly to a process which involves the simultaneous comminution of raw material and extraction of particles containing a high proportion of precious and heavy minerals from the raw material to facilitate the recovery of such metals and minerals.
Comminution is an essential step in the extraction of metals from their ores and from scrap materials containing such metals. Gold, for example, occurs in quartz veins or lodes and in alluvial placer deposits. Gold is extracted from its ores by comminuting the ores into particles which can be treated by chemical processes such as amalgamation, chlorination and dissolution in cyanide for the extraction of gold.
Comminution of ores or scrap materials can be carried out by conventional methods such as crushing, grinding and milling. A more recent method of comminution involves propelling particles of the material against a wall with sufficient force to cause the material to shatter into small particles. U.S. Pat. Nos. 5,839,671 and 6,024,307 issued on Nov. 24, 1998 and Feb. 15, 2000, respectively, both to Sand et al. describe the latter method.
Following comminution, the resulting particles must be further concentrated to remove the valuable metals and heavy minerals from them. Concentration is usually carried out in two or more stages and is accomplished by leaching, by flotation, by magnetic separation and so on.
We have found that comminution and a first stage of concentration of ores and scrap material containing precious metals and heavy minerals be carried out virtually simultaneously. If such material is propelled against a wall with sufficient force the material shatters into particles that can be classified in a stream of air which rises rapidly adjacent to the wall. Surprisingly, particles having a high proportion of precious metals and heavy minerals tend to hug the wall and fall while all other particles tend to be carried upwardly in the stream of air. This is unexpected; what would be expected is that the weight of the particles would govern whether they rise or fall. However what, in fact, governs this is the degree of purity of precious metals or heavy minerals in the particles. Light, smaller particles rich in precious metals or heavy minerals fall while heavy relatively large particles are carried upwardly in the stream of air.
Particles which fall in the stream of air contain little gangue and can be further concentrated at significantly less cost that particles of the same size from a conventional crushing or grinding operation. That is because particles from a conventional crusher or grinder generally contain significantly more gangue and such gangue adds to the overall cost of concentration.
Briefly, the process of our invention involves the steps of:
(i) providing a comminuting chamber having an interior space defined, in part, by side and lower walls;
(ii) directing raw material containing precious metals and heavy minerals downwardly onto impelling means located within the chamber such that the raw material collides with the impelling means and is driven thereby against the side wall with sufficient velocity to cause the raw material to fracture into a first fraction of particles which contain a substantial proportion of precious metals and heavy minerals and which are of a size in the range at which any precious metals and heavy minerals therein are substantially liberated from the remainder of the raw material, a second fraction of particles which are of a size in the same range but which contain substantially less precious metals and heavy minerals and a third fraction of particles which are of a size larger than the range;
(iii) causing air to flow into the chamber and along a path including: upwardly along the side wall;
(iv) recovering particles which collect on the lower wall for separation of precious metals and heavy minerals therefrom.
For the recovery of gold, my process briefly involves the steps of directing raw material containing gold downwardly onto impelling means located within the chamber such that the raw material collides with the impelling means and is driven thereby against the side wall with sufficient velocity to cause the raw material to fracture into a first fraction of particles which contain a substantial proportion of gold and which is of a size in the range at which any gold therein is substantially liberated from the remainder of the raw material, a second fraction of particles which are of a size in the same range but which contain substantially less gold and a third fraction of particles which are of a size larger than the range; causing air to flow into the chamber and along a path including upwardly along the side wall; and recovering particles which collect on the lower wall for separation of gold therefrom.